The production of three-dimensional articles of complex shape by means of stereolithography has been known for a relatively long time. In this technique the desired shaped article is built up from a liquid, radiation-curable composition with the aid of a recurring, alternating sequence of two steps (a) and (b); in step (a), a layer of the liquid, radiation-curable composition, one boundary of which is the surface of the composition, is cured with the aid of appropriate radiation, generally radiation produced by a preferably computer-controlled laser source, within a surface region which corresponds to the desired cross-sectional area of the shaped article to be formed, at the height of this layer, and in step (b) the cured layer is covered with a new layer of the liquid, radiation-curable composition, and the sequence of steps (a) and (b) is repeated until a so-called green model of the desired shape is finished. This green model is, in general, not yet fully cured and must therefore, normally, be subjected to post-curing.
The mechanical strength of the green model (modulus of elasticity, fracture strength), also referred to as green strength, constitutes an important property of the green model and is determined essentially by the nature of the stereolithographic-resin composition employed. Other important properties of a stereolithographic-resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum curl factor, permitting high shape definition of the green model. In addition, for example, the precured material layers should be readily wettable by the liquid stereolithographic-resin composition, and of course not only the green model but also the ultimately cured shaped article should have optimum mechanical properties.
Another requirement that has recently become a high priority for stereolithography users is the high temperature performance of cured articles produced by stereolithography. It is usually measured by the Heat Deflection Temperature (HDT) or Glass Transition Temperature (T.sub.g). The HDT value is determined by the ASTM method D648 applying a load of 66 psi.
For several years, high temperature performance for stereolithography produced articles has been achieved by the use of (meth)acrylate chemistry. This approach primarily entails the use of various commercially available urethane acrylate derivatives. EP-802455 of Teijin Seiki Corp. (Oct. 22, 1997) and JP 08323866 of Takemoto Oil & Fat Co Ltd (Dec. 10, 1996) describe acrylate urethane compositions for achieving good heat resistance and strength. However, a major disadvantage of such acrylate urethane compositions is that polymerization is hindered by atmospheric oxygen because polymerization is of a radical nature, that the cure shrinkage is unacceptably large, that the resins are irritant to the skin, particularly when the viscosity is low (low viscosity is highly preferred for stereolithography applications). Thus, acrylate urethane-based compositions show poor practicality for stereolithography.
Liquid, radiation-curable compositions for stereolithography which overcome the abovementioned problems of the acrylate chemistry are described, for example, in U.S. Pat. No. 5,476,748, which is incoporated herein by reference. These compositions are so-called hybrid systems, comprising free-radically and cationically photopolymerizable components. Such hybrid compositions comprise at least:
(A) a liquid difunctional or more highly functional epoxy resin or a liquid mixture consisting of difunctional or more highly functional epoxy resins; PA0 (B) a cationic photoinitiator or a mixture of cationic photoinitiators; PA0 (C) a free-radical photoinitiator or a mixture of free-radical photoinitiators; and PA0 (D) at least one liquid poly(meth)acrylate having a (meth)acrylate functionality of more than 2, PA0 (E) at least one liquid cycloaliphatic or aromatic diacrylate, and PA0 (F) a certain hydroxy compound that is selected from the group consisting of OH-terminated polyethers, polyesters and polyurethanes. Such hybrid systems can optionally further contain vinyl ether-based resins or other cationically cured components such as oxetanes, spiro-ortho esters. PA0 a) treating a radiation-curable composition described above with actinic radiation to form an at least partially cured layer on the surface of said composition within a surface region corresponding to a desired cross-sectional area of the three-dimensional article to be formed, PA0 b) covering the at least partially cured layer produced in step a) with a new layer of said radiation-curable composition, and c) repeating steps a) and b) until an article having the desired shape is formed, and optionally, d) post-curing the resulting article. PA0 A.sup.- is CF.sub.3 SO.sub.3.sup.- or an anion of the formula [LQ.sub.mB ].sup.-, where PA0 L is boron, phosphorus, arsenic or antimony, PA0 Q is a halogen atom, or some of the radicals Q in an anion LQ.sub.m.sup.- may also be hydroxyl groups, and PA0 mB is an integer corresponding to the valency of L enlarged by 1. PA0 dB is 1, 2, 3, 4 or 5, PA0 X.sub.B is a non-nucleophilic anion, especially PF.sub.6.sup.-, AsF.sub.6.sup.-, SbF.sub.6.sup.-, CF.sub.3 SO.sub.3.sup.-, C.sub.2 F.sub.5 SO.sub.3.sup.-, n-C.sub.3 F.sub.7 SO.sub.3.sup.-, n-C.sub.4 F.sub.9 SO.sub.3.sup.-, n-C.sub.6 F.sub.13 SO.sub.3.sup.- and n-C.sub.8 F.sub.17 SO.sub.3.sup.-, PA0 R.sub.8B is a .pi.-arene and PA0 R.sub.9B is an anion of a .pi.-arene, especially a cyclopentadienyl anion. PA0 Y.sub.F is a direct bond, C.sub.1 -C.sub.6 alkylene, --S--, --O--, --SO--, --SO.sub.2 -- or --CO--, PA0 R.sub.2F is a C.sub.1 -C.sub.8 alkyl group, a phenyl group which is unsubstituted or substituted by one or more C.sub.1 -C.sub.4 alkyl groups, hydroxyl groups or halogen atoms, or is a radical of the formula --CH.sub.2 --OR.sub.3F in which PA0 R.sub.3F is a C.sub.1 -C.sub.8 alkyl group or phenyl group, and PA0 A.sub.F is a radical selected from the radicals of the formulae ##STR5##
A drawback of commercial cationic or hybrid cationic-radical stereolithographic compositions is that their cured articles show HDT values that are much lower than those based on acrylate chemistry, usually between 40 and 100.degree. C.
Many hybrid compositions have been developed by companies for use in stereolithography process systems. For example, U.S. Pat. No. 5,434,196, assigned on its face to Asahi Denka Kogyo K. K., discloses resin compositions for so-called optical molding containing a mixture of epoxy resins and vinylethers, a cationic initiator, and a mixture of an acrylate compound and a triacrylate compound.
To date, there is no scientifically published or universally accepted term for defining the high temperature hybrid stereolithography resins. Through marketing brochures of stereolithography resin maufacturers and presentations in trade organizations, high temperature hybrid stereolithography resins are defined as those wherein their cured articles have HDT values over 80 and about 100.degree. C. The highest HDT value ever reported for commercial hybrid stereolithography resins is about 100.degree. C.
Despite all previous attempts, there exists a need for hybrid stereolithography compositions capable of producing high temperature performance cured articles for which the photospeed, accuracy, water resistance are commercially acceptable. Such hybrid compositions should possess HDT values over those of the existing ones, especially over 110.degree. C.